In the field of magnet-type machines with small dimensions, typically, but not limitatively, with a diameter of less than 30 mm, it is common to use slotless stator topologies rather than slotted topologies, corresponding to a stator consisting of a plurality of teeth, wound or not. Slotless solutions are particularly suitable for small diameters, in particular when it is wished to obtain a very low or even zero current-free torque, which proves to be very difficult with slotted structures whether because of the stator circuit (geometric tolerances) or with regard to the rotor (geometric tolerances or homogeneity of the permanent magnets). More particularly, a slotless machine having a homogeneous magnetic air gap has a zero current-free torque. Because of this, such a machine is particularly favourable for use at very high speed (typically from a few tens of thousands to a few hundreds of thousands of revolutions per minute) since the absence of current-free torque minimises torque ripple, vibrations, noise, losses, etc.
On the other hand, the absence of stator teeth reduces the power density of such machines, and reduces the relevance thereof for use in large-diameter structures and/or ones intended to produce high mechanical powers at low speed. The winding is a particularly critical problem in slotless motors, in particular in the case of extreme miniaturisation, typically for diameters of less than 10 millimetres. By way of example, the functioning of such motors is described in the article by G. J. Yan, J. H. Wang & S. J. Yan, J. Sci. Innov., 2012, Vol. 2, No. 1, 39-48.
Self-supporting circumferential windings (or “bell” windings) traditionally used in these slotless motors are described for example in the patents GB 903285 and GB 1046993. These solutions give rise to high manufacturing costs, in particular when it is wished to produce a motor with a very small diameter. Such a winding is also not suitable for configurations using a small number of turns of wire with a large diameter, as is frequently the case when it is sought to achieve high speeds.
Solutions are known proposing a concentrated winding (a winding where the turns are contiguous) but based on separate coils, referred to as self-supporting (also referred to as “air core”, that is to say wound individually and without any support, in particular a ferromagnetic one) that it is necessary to position in the stator and then fix, often via the use of overmoulding, and finally to isolate from the stator. This involves expensive and complex implementations.
To remedy this, the patent application JP 2008/154340 proposes the use of non-magnetic shells in the form of a cross-section of a cylinder each carrying a self-supporting coil positioned by means of one or more protrusions extending radially inwards, the coils then being held in place by resin, and the shells finally being connected together. This implementation process is complex and connecting together a plurality of parts in a micromotor requires very strict tolerances in design and positioning of the parts. Another solution described in the patent application JP 2004/254443 describes the use of an articulated support, placed flat, and on which the coils are wound by an automated process, said support then being coiled and inserted in the motor housing. Such a solution is not suited to motors of very small sizes since the extremely small thicknesses of material at the articulation between each zone supporting the coils makes the assembly extremely fragile.
To remedy certain defects described above, the use of a single piece on which the coils are wound is an advantageous solution. The patent application JP 2010/242407 illustrates a variant thereof where three coils are wound from the outside on a cylinder extended by three protrusions extending radially outwards and serving as supports for the winding. However, such a solution requires the addition of an insulator between the winding and the flux-closure part enclosing the winding. In addition, in very small diameters (typically below 10 mm), the thickness of these protrusions becomes a real problem.
In the prior art the American patent application US 2012/0274167 is known, describing a DC brushless motor having a stator without grooves and more specifically a DC brushless motor having a stator without grooves designed to avoid the presence of forces interfering with a rotation in the rotor by the formation of a shuttle rotor from an insulating material while simultaneously allowing supply to the DC motor at high rate; the embodiment according to this document of the prior art relates to the winding of a large quantity of coils in the direction of the length of a rotary shaft by forming a plurality of winding projections that make it possible to wind the coils on a shuttle rotor, so that the winding protuberances project with spaces at predefined intervals along the circumference of the external peripheral surface of the shuttle rotor body. Once again, the use of such a solution in small diameters is problematic.
The solutions of the prior art do not make it possible to achieve optimum performances (speed, power). Moreover, the configuration of the stators makes it difficult to carry out winding with wires with a diameter suited to high powers. In particular, for motors intended for high speeds, several hundreds of thousands of revolutions per minute, it is necessary to reduce the number of turns per coil and per motor phase, and therefore to increase the cross-section of the winding wires. In general, the solutions proposed in the prior art are not suited to the case of micromachines, typically with a diameter of less than 10 mm.
Thus the solution proposed by the American patent application US 2012/0274167 provides a large number of coils, leading to a large outside diameter. Moreover, this solution involves the use of additional insulation parts and grooving of the external envelope.